The background of the invention is set forth in two parts: the field of the invention and the description of related art.
1. Field of the Invention
This invention relates to a method and apparatus for pressure and level transmission and sensing. In particular, it relates to a method and apparatus for transmitting and sensing liquid pressure (head) and liquid level (depth) in flow-metering, tide gauge, and liquid or slurry density-measurement applications.
2. Description of Related Art
Accurate and reliable (stable) measurement of liquid pressure (head) and/or liquid level over a wide temperature range and over an extended period of time is required in a variety of applications. Flow metering applications typically comprise a primary flow metering element or device and a secondary flow metering element or device. The primary flow metering element is a constriction that causes there to be a unique relationship between liquid pressures and/or liquid levels and the flow rate. The secondary flow metering element typically accomplishes three functions: (1) measuring the pressures and/or levels produced by the primary element, (2) converting these measurements into flow rate data, and (3) recording the flow rate data.
A variety of constrictions are incorporated into primary elements. For the purposes of this disclosure, the term "constriction" means a reduction in the cross-sectional area of a conduit. The reduction may be gradual or abrupt. An example is given by W. H. Hager in "Venturi Flume of Minimum Space Requirements" in Journal of Irrigation and Drainage Engineering, Vol. 114, No. 2, May, 1988, p. 226, which example comprises "sharp-edged constriction plates placed symmetrically in a rectangular channel." Yet another example is given in U.S. Pat. No. 4,571,997 by Kepple et al., Feb. 25, 1986, which example comprises a metering insert that has been commercialized under the trade name "Flowpoke". Another example is given by F. A. Kilpatrich, W. R. Kaehrle, J. Hardee, E. H. Cordes and M. N. Landers in Development and Testing of Highway Storm-Sewer Flow Measurement and Recording System, U.S. Geological Survey Water-Resources Investigations Report 85-4111, 1985, which example comprises a Palmer-Bowlus flume installed in a closed conduit.
In open-channel flow metering, accurate measurement of the water levels upstream and downstream from the primary flow metering element (e.g., flume or weir) is necessary. For example, a water level measurement accuracy of .+-.3 millimeters (mm) of water is generally required for accurate flow metering of flow rates of 0.4 cubic meters per second (14 cubic feet per second) or less. In closed conduit flow metering, accurate measurement of pressures (heads) in the inlet and in the throat and/or outlet of the primary flow element (e.g., venturi tube or orifice) is required.
In tide gauge applications, accurate and stable measurement of water level is required. The U.S. National Ocean Service, for example, has indicated that it wants to be able to gauge tidal variations with an accuracy of .+-.3 mm and an annual stability of .+-.3 mm. Moreover, a production cost of the sensor (without the data recording system) should be $1,000 or less (in 1991 U.S. dollars).
Bubbler-type systems have also been used to measure density or salinity. The differential pressure (p) between two bubbler discharge ports located a known vertical distance (h) apart is used to determine the average density (w) of the liquid between the ports. The average density of the fluid between the bubbler discharge ports is given by the formula: w=p/h.
A variety of techniques have been used to measure water level or head at a point in a liquid, but none is both low in cost and accurate. Microwave technology can be used to sense water level with an accuracy of .+-.25 mm. Ultrasound technology can be used to sense water level within accuracy of .+-.0.25 percent of full scale (F.S.) or .+-.5 mm, whichever is greater. Capacitance technology can be used to sense water level with an accuracy to .+-.1-2 percent of F.S.
A variety of bubbler-type pressure transmitting and sensing systems have been disclosed in prior art references. Bubbler-type systems for level sensing are disclosed in U.S. Pat. Nos. 3,620,085 by Khoi, Nov. 16, 1971; 4,526,035 by Auchapt et al., Jul. 2, 1985; 4,625,548 by Charter, Dec. 2, 1986; 4,711,127 by Hafner, Dec. 8, 1987; 4,719,799 by Wicks et al., Jan. 19, 1988; 4,869,104 by Saito et al., Sep. 26, 1989 and 5,052,222 by Stoepfel, Oct. 1, 1991. Bubbler-type systems for density sensing are disclosed in U.S. Pat. Nos. 2,557,548 by Vetter, Dec. 4, 1951; 2,604,778 by Marquardt, Jul. 29, 1952; 2,668,438 by Marquardt, Feb. 9, 1954; 2,755,669 by Beard, Jul. 24, 1956; 3,380,463 by Trethewey, Apr. 30, 1968; 3,399,573 by Ponsar, Sep. 3, 1968; 3,460,394 by Cryer, Aug. 12, 1969; 3,613,456 by Hoppe et al., Oct. 19, 1971; 4,307,609 by Rosenblum, Dec. 29, 1981; 4,393,705 by Eidshun, Jul. 19, 1983; 4,419,893 by Baillie et al., Dec. 13, 1983; 4,485,675 by Verret, Dec. 4, 1984; 4,949,572 by Wilen et al., Aug. 21, 1990 and 5,020,368 by Evans et al., Oct. 1, 1991. Bubbler-type systems for flow metering are disclosed in U.S. Pat. Nos. 4,367,652 by Venuso, Jan. 11, 1983; 4,388,827 by Palmer et al., Jun. 21, 1983 and 4,669,308 by Jorritsma, Jun. 2, 1987.
Bubbler-type pressure transmitting and sensing systems are also disclosed by A. R. Dedrick and A. J. Clemmens in "Double-Bubblers Coupled with Pressure Transducers for Water Level Sensing" in Transactions of the ASAE, 1984, p. 779, and A. R. Dedrick and A. J. Clemmens in "Instrumentation for Monitoring Water Levels" in Proceedings of the Agri-Mation.TM.2 Conference & Exposition, Mar. 3-5, 1986, Chicago, Ill., p. 148. These systems implement the "double-bubbler" concept in which the pressure in two bubbler tubes is determined sequentially by means of a gauge pressure sensor referenced to atmospheric pressure. The ends of the bubbler tubes are submerged in water and are located a known vertical distance apart in fresh water. The difference between the two sequential pressure measurements comprises a known head that is used to calibrate the gain (span) of the pressure sensor. In that the gauge pressure sensor can be zeroed by venting its pressure port to atmospheric pressure, both the offset and span of the sensor can be recalibrated prior to use of the pressure sensor to measure the unknown pressure in one of the bubbler tubes. Limitations of this system include the time required for sequential exposure of a gauge pressure sensor to the pressures in two different bubbler circuits to reset gain and incorporation of bulky and heavy needle valves and differential pressure regulators into the system.